The present invention relates to the manufacture of tires. More particularly, it concerns the assembling of tires on a drum.
In this field, so-called "shaping" drums, in particular, are known, that is to say drums having means which permit the bringing together of the bead recesses of the tire which are provided on the drum, used together with means for effecting a radial expansion of all or part of the tire blank produced on said drum.
In a first stage of the manufacture of a tire on a drum of this type, a quasi-cylindrical carcass sleeve is produced by winding onto the outer surface of the drum, in the order required by the structure of said tire, numerous semifinished products comprising either mixes having a base of unvulcanized rubber or reinforcement elements such as cords or cables coated with mixes having a base of unvulcanized rubber, generally prepared in the form of plies.
After having put the bead wires of the tire in place on the sleeve, facing the bead recesses provided in the building drum, parts of the sleeve which are located radially below the bead wires are expanded radially until said parts are blocked below the bead wires. The portion of the sleeve located between the bead recesses is then deformed in order to obtain a toroidal shape, while axially bringing the said recesses towards each other. The clamping of the sleeve under the bead wires must be sufficient to prevent any relative displacement of the sleeve with respect to the bead wires during this operation.
This radial deformation is obtained, for instance, by the inflating of a rubber bladder surrounding the building drum between the axially innermost bead recesses and permitting the laying of the products forming the tire on the quasi-cylindrical outer surface of said bladder when it is deflated.
Almost simultaneously with this second stage, the portions of the sleeve which are furthest axially to the outside are themselves also expanded radially towards the outside by the use of means forming part of the building drum and are progressively applied against the expanded portion of the sleeve located between the bead recesses. This operation, during which the beads are first formed and then the sidewalls of the tire, is known as "turning-up".
U.S. Pat. No. 4,683,021 describes a tire-building drum in which the turning-up of the portions of the sleeve furthest axially to the outside is effected by the inflating of a first bladder of reduced dimensions for the forming of the beads, followed by the inflating of a second bladder which surrounds the first and is of much larger dimensions in order to form the sidewalls of the tire up to the points of said tires furthest radially to the outside; furthermore, another bladder is provided which is located axially to the outside of the two preceding ones in order to be able to effect the putting in place of the portion of the sidewall which is radially outermost by exerting an inwardly directed axial push on each of the turn-up bladders.
However, such a process does not assure an excellent production of the beads since the turn-up bladder, although of small dimensions, cannot produce an axial force sufficient to avoid the formation of air pockets in said beads, which can prove harmful with respect to the overall strength of the tire during its subsequent use; furthermore, the operation of the turning-up of the rubber-base products constituting the sidewalls of the tire requires the use of bladders of such dimensions that, once inflated, said bladders have their radially furthest points from the axis of symmetry of the building drum located on a radius at least equal to that of the points of the sidewalls of the tire which are furthest radially to the outside. Because of this, it results that the products which have been placed on this bladder in its deflated state will have a tendency to slide radially towards the inside and towards the beads during the turning-up phase, since they are entirely on the same side of the inflated bladder with respect to the radially outermost point of said bladder. There results from this an extensive dispersion of the heights of the products placed on the sidewalls of the tire and possibly the formation of folds, with an unfavorable effect on the overall strength of said tire.
U.S. Pat. No. 2,435,345 teaches that a single turn-up bladder at each end of the building drum can be used in order to form the beads and the sidewalls of the tire simultaneously, provided that means permitting the exerting of an axial thrust on said bladders is added to each of these bladders. We may note that each outside bladder, as a result of its shape, does not permit, any more than in the previously cited patent, a good holding of the products to be turned-up up to the zone of connection with the crown of the tire; finally, the said axial thrust means lead to an additional space requirement which is particularly disturbing in an industrial context.